I would alway recommend a 85 to 115 amp hour leisure battery. with a split charge relay and boost charged every couple of days (depending on mileage travelled) with an intelligent battery charger.
There are some on here who have successfully used separate car batteries and others who have successfully used their own van batteries. I would imagine your Focus battery is a reasonable size, so how have you managed so far and why do you need to change if you don't mind me asking?
One of the other local cleaners has used 2 car batteries for years but they aren't lasting the day any more. He puts it down to the cold weather, but then a battery doesn't last forever.
Spruce